Question: How Does VoIP Allow Calls Between IP Networks And PSTN?
With VoIP, you use an IP network, through an ADSL or other Internet connection, to make/receive phone calls to/from PSTN landline networks. You can even talk to people with a landline phone number through your computer, for example using SkypeIn and SkypeOut service. PSTN has phone numbers and IP networks have IP addresses. How can these two work together?
Answer: The answer is address translation. Your computer or VoIP device connected to the Internet has an IP address assigned to it. When you call a person on a PSTN landline phone, your call is handed over to the PSTN network through the person's phone number, which you dial. And vice-versa.
In VoIP, every phone number has an IP address to which it maps. Each time a device (PC, IP phone, ATA etc.) engages in a VoIP call, its IP address is translated into the phone number, which is then handed over to the PSTN network. This is analogous to the way web addresses (domain names) and email addresses are mapped to IP addresses.
This is besides the basis of some of the patents that Verizon claimed Vonage infringed upon in March 2007. This caused big trouble for Vonage, one of the leading VoIP companies. Vonage used the PSTN network to translate VoIP addresses (and hence numbers) to. Since Verizon has some history and certain rights over PSTN, they sued Vonage, and won their case.
